The potential punishment for an av melacha on Shabbos is stoning (ch"v). (Obviously, only under certain circumstances and after repeated attempts to dissuade the sinner.) However, I learned here that the potential punishment for doing an [av] melacha on Yom Kippur is only kareit. Kareit is the fifth of Rambam's eight levels of punishment, whereas death by stoning is the first. This is a big difference in strictness.
I understand that in practice in modern times, this doesn't change anything. However, if Yom Kippur is the Shabbas Shabbason and overrides Shabbat in almost every other way(1), why would the punishment for melacha on Yom Kippur be so much less severe?
Related: Melachas on Yom Kippur
- Lopatin, Rabbi Asher. "Celebrating a Conflicted Relationship with God." In Naming God: Avinu Malkeinu— Our Father, Our King, edited by Rabbi Lawrence A. Hoffman, PhD, 231-35. Prayers of Awe. Woodstock, VT: Jewish Lights Publishing, 2015.